(1) Field of the Invention
The present invention relates to the field of rotorcraft, and more particularly to automatic flight control systems that have an influence on the flight behavior of the rotorcraft. The automatic flight control system for a rotorcraft to which the present invention relates is an automatic system for providing the pilot of the rotorcraft with assistance in performing difficult procedures acting on the flight behavior of the rotorcraft, in particular at low speeds, such as speeds of less than about 50 knots (kts).
(2) Description of Related Art
Rotorcraft are aircraft that differ from other powered aircraft mainly in their ability to fly both at high cruising speeds and to fly at low speeds or to hover. This ability is obtained by providing the rotorcraft with at least one main rotor having an axis that is substantially vertical.
The main rotor comprises a rotary wing that provides the rotorcraft with lift and possibly also with thrust. The behavior of the rotorcraft in flight is modified by varying the cyclic and/or collective pitch of the blades of the rotary wing. A variation in the cyclic pitch of the blades leads to a change in the attitude of the rotorcraft, and more particularly to a change that can be varied between pitching and/or rolling. A variation in the collective pitch of the blades gives rise to a modification to the lift behavior of the rotorcraft, and more particularly behavior along the gravity axis.
When the main rotor provides lift only, and possibly also with marginal propulsion as happens with a hybrid helicopter, the rotorcraft is fitted with means specifically for providing propulsion. For example, hybrid helicopters have a main rotor serving to provide the major portion of the lift of the rotorcraft and, to a lesser extent, its propulsion, together with at least one propulsive propeller.
The rotorcraft is also maneuverable in yaw about its own vertical axis, by making use of a yaw anti-torque device. For example, such an anti-torque device is formed by an anti-torque rotor having an axis that is substantially horizontal compared with the substantially vertical axis of the main rotor. By way of example, the anti-torque rotor is a tail rotor of the rotorcraft, or, by way of example and in a hybrid helicopter, it is formed by at least one of the propulsive propellers fitted to such a hybrid helicopter. With an anti-torque rotor, varying the collective pitch of the blades of the anti-torque rotor gives rise to a variation in the yaw progression of the rotorcraft.
The specific ability of a rotorcraft to fly at low speeds or to hover implies a special organization for the means used to manage its flight behavior. The difficulties associated with such a special organization need to be considered independently of potential use of said means for high-speed progression of the rotorcraft.
A variation in the flight behavior of the rotorcraft is achieved by modifying flight parameters of the rotorcraft, which parameters are defined relative to cyclic and/or collective pitch values for the main rotor and/or collective pitch values for the anti-torque rotor. Such a modification to the flight parameters may be achieved using various control modes.
In a manual control mode, the pilot of the rotorcraft has manual control members that are moved by a person in order to vary the pitch of the blades or of the rotors by means of manual control linkages respectively allocated to each of the progression axes of the rotorcraft. The concept of “manual” should be understood as being in opposition to the concept of “automatic”, without prejudice to the means actually used by a person for controlling the rotorcraft, in particular a hand-operated stick or foot-operated rudder pedals.
In an embodiment of a manual control mode, the manual control members are engaged with respective linkages for transmitting forces remotely, enabling the pilot of the rotorcraft to act mechanically on the blades by means of manual control members, either directly or else via servo-controls for a heavy helicopter.
In another embodiment of a manual control mode, movement of a manual control member by the pilot generates electrical signals for activating at least one blade-moving servo-control.
In an automatic control mode, an autopilot generates commands automatically for moving the blades by means of automatic control linkages allocated respectively to each of the progression axes of the rotorcraft. When the autopilot is activated, the automatic commands take the place of the commands generated by the pilot directly on the manual control members in order to activate the servo-controls.
The autopilot serves to maintain stable progression of the rotorcraft in compliance with a previously-stored setpoint. An actual state of progression of the rotorcraft is evaluated by the autopilot at a given instant, according to information supplied by onboard instrumentation of the rotorcraft. On the autopilot detecting a difference between the setpoint and the actual state of progression of the rotorcraft, the autopilot intervenes on the flight behavior of the rotorcraft in order to bring its actual state of progression back into conformity with the setpoint.
Activation of the autopilot is controlled by the pilot of the rotorcraft using one or more specific control knobs.
In a stabilization mode performed by the autopilot, an initial setpoint for maintaining the attitude of the rotorcraft is defined relative to the state of progression of the rotorcraft as evaluated on activating the autopilot. The stabilization mode stabilizes the rotorcraft by correcting the attitude of the rotorcraft by means of the autopilot acting relative to the initial setpoint.
On this topic, reference may be made for example to document FR 1 347 243 (Boeing Co.), which discloses ways of stabilizing the behavior of a rotorcraft by an autopilot by holding to previously-stored flight setpoints. The detection of a deviation of the rotorcraft from its path generated error signals, and flight commands are issued by the autopilot to correct the attitude of the rotorcraft until the error signals are reduced to a value of zero. The flight commands are issued by the autopilot for at least one of the progression axes of the rotorcraft in a manner that is mutually synchronized for all of the progression axes so that the behavior of the rotorcraft is kept stable in the event of the position of the rotorcraft being corrected relative to any one of the progression axes.
In a particular mode of piloting by transparency, the pilot of the rotorcraft may optionally act temporarily on the behavior of the rotorcraft by using the manual control linkages, thereby overruling the commands generated by the autopilot. The initial setpoint is left unchanged, and any temporary action on the part of the pilot on the behavior of the rotorcraft does not lead to any modification of the initial setpoint.
It is also known to correct an initial setpoint for maintaining attitude as a function of the actual state of progression of the rotorcraft as evaluated after the pilot has operated the manual control members. It is also known to enable the pilot of the rotorcraft to correct an initial setpoint for maintaining attitude by varying its values incrementally.
On this topic, reference may be made for example to the document GB 809 278 (Bendix Aviat Corp.), which describes such ways of correcting setpoints for maintaining attitude. More particularly, an autopilot maintains the positions of the elevators and the ailerons of an airplane in compliance with flight setpoints to achieve progression of the airplane that is stabilized in pitching, in roll, and in yaw. A human pilot can act on the progression of the airplane by modifying the positions of the ailerons, while conserving autopilot stabilization of the behavior of the airplane. At the end of the action taken by the human pilot, and once the attitude of the airplane is stabilized at the desired altitude, the flight setpoints are conserved in the current state of progression of the rotorcraft.
Stabilization of the rotorcraft is achieved using basic modes in which the autopilot acts e.g. to generate increased stability by damping angular movements of the rotorcraft, or indeed to maintain attitudes or a heading, or indeed to decouple progression axes, for example. These basic modes provide piloting comfort for the pilot of the rotorcraft, but they do not correct possible deviations of position of the rotorcraft. Proposals have therefore been made to associate higher modes of operation with such basic modes in order to eliminate possible deviations of position, of speed, and/or of acceleration of the rotorcraft. The behavior of the rotorcraft is managed by the autopilot as a function of the flight setpoint so as to keep the rotorcraft stable and so as to reestablish its position, its speed, or its acceleration by using the higher modes. The autopilot performs the operation of stabilizing the rotorcraft quickly by using the basic mode, whereas it subsequently performs the operation of reestablishing the position, the speed, and/or the acceleration of the rotorcraft more slowly by using the higher modes.
The autopilot is also capable of performing advanced functions of assisting the guidance of the rotorcraft. The facilities potentially made available by the higher modes are used in auxiliary manner to achieve such assistance. Various advanced functions may be used to achieve assistance in the guidance of the rotorcraft. The ways in which advanced functions are performed relate to predefined functionalities of the autopilot relating to a path to be followed by the rotorcraft.
In various advanced functions of the autopilot making use of the higher modes, the rotorcraft is guided by the autopilot relative to a previously-defined setpoint path. The autopilot can then make use of various geolocation means for guiding the rotorcraft along a setpoint path.
By way of example, the setpoint path is used relative to a flight mission as previously determined by the pilot of the rotorcraft, or it is used during a stage of approaching a known and identified site. In particular, such a site is fitted with means that provide interaction between the site and the autopilot, such as radio navigation beacons. In the absence of such interactive equipment, site identification is performed by the pilot of the rotorcraft in manual mode, and then the pilot of the rotorcraft activates the desired advanced function.
For such advanced functions making use of the higher modes of the autopilot, two superposed loops are used for servo-controlling flight parameters. A fast servo-control loop is used for correcting attitude, yaw, or verticality of the rotorcraft. A slow servo-control loop is used by the higher mode for reducing any guidance deviation of the rotorcraft to zero.
The autopilot is conventionally in communication with display means that provide the pilot of the rotorcraft with various kinds of information, e.g. such as information about various flight parameters, about the flight mission to be performed by the rotorcraft, about weather conditions, and/or about the environment outside the rotorcraft, or indeed about the environment at the site of intervention. Such information is useful for controlling the rotorcraft by the autopilot and/or by the pilot of the rotorcraft.
On this topic, reference may be made for example to the document US 2011/137492 (Sahasrabudhe Vineet et al.), which discloses ways of automatically guiding a rotorcraft along a path by making use of said advanced functions (a vertical takeoff and landing (VTOL) function). Guidance of the rotorcraft is performed in flight using instruments, while taking account of an actual state of progression of the rotorcraft and a state of progression that the rotorcraft is to achieve so as to be guided along a predefined approach path. The human pilot has information display means available that display information about guiding the rotorcraft along the path relative to the outside environment.
The ways in which the autopilot operates provide automatic assistance to piloting that is satisfactory in terms of correcting the attitude of the rotorcraft in a cruising stage of flight, at high speed, and while the rotorcraft is far away from the ground. During a cruising stage of flight, the surroundings of the rotorcraft are normally empty, and the pilot of the rotorcraft does not need to concentrate on maneuvering the rotorcraft. It can also happen that there is no need for such concentration when close to the ground and in a known environment, with this being made possible by using an advanced function of the autopilot, such as during a stage of approaching a known runway and/or a runway that is fitted with means for identifying its environment.
Automatic assistance obtained by the autopilot performing an advanced function can be satisfactory during a stage of approaching an intervention site, including at low speed, providing the intervention site is well known, identified, and indicated to the autopilot. Once the intervention site has been identified, it is possible to activate an advanced function in order to guide the rotorcraft along the corresponding setpoint path.
In general, rotorcraft are powered aircraft that are designed to be used under flight conditions that are difficult, such as at low speeds or while hovering, close to the ground anywhere, in a position that may be unknown or poorly known, and with arbitrary conditions of visibility and/or an environment that is hostile and/or unknown.
Under difficult flying conditions, unexpected factors might need to be taken into account by the pilot of the rotorcraft. It is difficult or even impossible for the pilot of the rotorcraft to make use of automatic assistance in maneuvering the rotorcraft under such difficult conditions. For example, when the rotorcraft is close to the ground, any change needed in its behavior must be performed quickly. The ways in which the autopilot operates make it difficult to act quickly to modify a path to be followed by the rotorcraft by making use of an advanced function that implements the higher modes.
Thus, a landing zone might be poorly known while the pilot of the rotorcraft is preparing a flight mission. Access conditions to the landing zone might initially be identified as being potentially difficult, or unknown, or indeed hostile. Access to the landing zone may also be made particularly difficult, since the real environment at the site might be different from and/or temporarily modified relative to the expectations of the pilot of the rotorcraft. Prior location of the landing zone using geolocation means can be approximate, and possibly even uncertain. Poor visibility does not make it any easier for the pilot of the rotorcraft to identify quickly on site the difficulties that need to be overcome in order to approach the landing zone.
Under such conditions, the pilot of the rotorcraft can become confused as to which automatic or manual control modes should be selected in order to approach a landing zone in a difficult flying situation. It is then found that the pilot of the rotorcraft needs to have available the advantages provided by piloting assisted by means of the autopilot, while still retaining the ability to intervene quickly in manual mode on the behavior of the rotorcraft.
It is useful to avoid the pilot of a rotorcraft becoming confused in this way during a difficult stage of approach to an intervention site, by enhancing the ways in which the autopilot operates so as to make it possible for the human pilot to intervene quickly on the behavior of the rotorcraft.